Milk thistle is an annual/biennial plant that grows in dry, rocky soil throughout southern and Western Europe, and in some parts of the United States. The branched stem grows up to three feet in height and bears spiny, scalloped-edged leaves, which are dark green in color and streaked with white along the veins. Milk thistle flowers from June to August, bearing a single spiny, reddish-purple blossom. The fruit of the plant (commonly misidentified as the seed) is used for health purposes.
The active components of milk thistle are a group of flavonoids known as silymarin, which consists of three compounds: silibinin, silydianin, and silychristin. Of the three, silibinin appears to exhibit the most biological activity. Milk thistle extracts are commonly standardized to contain 70 to 80 percent silymarin.
Studies on milk thistle have used doses ranging from 200 to 600 mg per day, and there have been no significant side effects reported in this dosage range. Milk thistle has no known toxicity, even in extremely high doses. To ensure consistent potency, most herbalists recommend milk thistle extracts that are standardized to contain 70 to 80 percent silymarin.